This invention pertains generally to the field of electronic assemblies and cooling systems therefor. While applicable to a variety of electronic fields, it is believed that this invention has its greatest applicability in the field of high speed, high capacity digital computers, sometimes referred to as supercomputers, and the description herein of the presently preferred embodiment of the invention illustrates the use of the invention in such a computer.
In the development of very high speed computers, great efforts have been directed toward reducing the physical dimensions of the computer assembly because signal propagation delays due to the maximum interconnect path length limit the maximum clock rate and hence speed of operation of the computer. Currently available generations of logic and memory integrated circuits are capable of switching at clock rates in the nanosecond range, but in order for such a rate to be used in the computer, the maximum length of the longest interconnecting path between circuits must be held to a very short distance, for example, about 16 inches length in the case of twisted wire pairs for four nanosecond operation.
Advances in integrated circuit technology have produced devices with increased numbers of logic gates and memory circuits per chip, making it theoretically possible to assemble the great number of logic and memory circuits needed for a supercomputer within an area permitting a 16-inch or comparably short wire length interconnect distance. Unfortunately, that theoretically possible high density cannot be achieved in practice unless the very considerable amount of heat generated by such a high density assemblage of circuits can be successfully removed. A single emitter coupled logic integrated circuit can dissipate as heat energy up to one watt of power. With high density packaging it is possible to put enough of such integrated circuits into a 1-inch by 4-inch by 8-inch module to generate 600 or 700 watts. When it is considered that many dozens of such modules would have to be placed close together to achieve the desired result, it can be appreciated that the amount of heat to be dissipated far exceeds available cooling techniques.
A number of techniques have been developed in the field of electronics for cooling electronic components and circuits. When air cooling and forced air cooling became inadequate, liquid or refrigerant filled cold bar or cold plate chassis members were developed for supporting circuit modules and conducting heat away from them. In U.S. Pat. No. 4,120,021 which is assigned to the assignee of this application, a cooling system is disclosed employing refrigerant cooled cold bars having slots and clamping means for receiving the edges of plates to which circuit boards are mounted. Heat generated by the circuit components is transmitted by convection and conduction to the cold plates and then to the cold bars. While this technique has been extremely successful for its intended purpose, it is insufficient for the extremely high density and heat loading described above.
Cooling of electronic components by immersion in inert liquid has been practiced in various forms in a number of areas of electronics. Inert liquids suitable for electronic immersion are available, for example, a fluorocarbon product called Fluorinert produced by the 3M Company. These liquids can be obtained with different boiling points to serve different needs. A common use has been the placing of a single component in such fluid for isolating it for testing purposes. High-powered rectifiers have also been immersion cooled. Computer circuit modules have been proposed in which a number of circuit boards have been mounted within a sealed container to form a module of a computer system. The modules are filled with inert liquid which removes heat from the circuits by nucleate boiling and recondensing on the walls of the module. The heat is then transferred to the surrounding air by fins formed on the housing of the module.
Immersion cooling has great advantages over air cooling in terms of higher heat transfer rate and higher heat capacity of liquid compared with gas. However, immersion of circuitry in fluid alone is not sufficient to solve the heat problems associated with the very high density, large scale systems discussed above. It is necessary to also provide for mechanical and electrical construction of the electronic assembly in a manner that permits very high density packaging and effective removal of heat from the components by the liquid, while still providing an assembly reasonably accessible for service or updates.